Um, Hello?
Prolactin, aside from bringing milk to the jugs, also (according to Wikipedia*):
Provide[s] the body with sexual gratification after sexual acts: The hormone counteracts the effect of dopamine, which is responsible for sexual arousal. This is thought to cause the sexual refractory period. The amount of prolactin can be an indicator for the amount of sexual satisfaction and relaxation.
But…
Unusually high amounts are suspected to be responsible for impotence and loss of libido (see hyperprolactinemia Symptoms).
Fail? “Unusually high” is pretty vague.
*I hate that Wikipedia has become some beacon of “truth” for netizens.